The luciferase secreted from the deep-sea shrimp Oplophorus gracilirostris has been shown to possess many interesting characteristics, such as high activity, high quantum yield, and broad substrate specificity (including, e.g., coelenterazine as well as various coelenterazine analogs). The bioluminescent reaction of Oplophorus takes place when the oxidation of coelenterazine (substrate) with molecular oxygen is catalyzed by Oplophorus luciferase, resulting in light of maximum intensity at 462 nm and the products CO2 and coelenteramide (Shimomura et al., Biochemistry, 17:994 (1978)). Optimum luminescence occurs at pH 9 in the presence of 0.05-0.1 M NaCl at 40° C., and, due to the unusual resistance of this enzyme to heat, visible luminescence occurs at temperatures above 50° C. when the highly purified enzyme is used or at over 70° C. when partially purified enzyme is used. At pH 8.7, the native luciferase was reported by Shimomura et al. (1978) to have a molecular weight of approximately 130 kDa, apparently comprising four monomers of 31 kDa each; at lower pH, the native luciferase tends to polymerize.
Later work has shown that the Oplophorus gracilirostris luciferase is a complex of native 35 kDa and 19 kDa proteins, i.e., a heterotetramer consisting of two 19 kDa components and two 35 kDa components. Inouye et al. (2000) reported the molecular cloning of the cDNAs encoding the 35 kDa and 19 kDa proteins of Oplophorus luciferase, and the identification of the protein component that catalyzes the luminescence reaction. The cDNAs encoding the proteins were expressed in bacterial and mammalian cells, and the 19 kDa protein was identified as the component capable of catalyzing the luminescent oxidation of coelenterazine (Inouye et al., 2000).
The 19 kDa protein of Oplophorus luciferase (GenBank accession BAB13776, 196 amino acids) appears to be the smallest catalytic component having luciferase function, and its primary structure has no significant similarity with any reported luciferase including imidazopyrazinone luciferases (Lorenz et al., PNAS USA, 88:4438 (1991); Thompson et al., PNAS USA, 86:6567 (1989)). Expression of the 19 kDa protein in E. coli resulted in the formation of inclusion bodies (Inouye and Sasaki, Protein Expression and Purification, 56:261-268 (2007)). The formation of inclusion bodies is likely due to the instability of the protein.
The substrate specificity of Oplophorus luciferase is unexpectedly broad (Inouye and Shimomura. BBRC, 223:349 (1997)). For instance, bisdeoxycoelenterazine (i.e., coelenterazine-hh), an analog of coelenterazine, is an excellent substrate for Oplophorus luciferase comparable to coelenterazine (Nakamura et al., Tetrahedron Lett., 38:6405 (1997)). Moreover, Oplophorus luciferase is a secreted enzyme, like the luciferase of the marine ostracod Cypridina (Vargula) hilgendorfii (Johnson and Shimomura, Meth. Enzyme, 57:331 (1978)), which also uses an imidazopyrazinone-type luciferin to emit light.